In modern mobile radio systems, such as in the Bluetooth system, the frequency is spread, in addition to time duplexing, “TDD”, Time Division Duplex, by transmitting and receiving on different channels, which is also referred to as frequency hopping, FH. This does not involve transmitting and receiving in a constant physical channel, but rather the frequency of the transmission channel is changed after every timeslot in the data transmission.
If some of these transmission channels contain spectral interference, then it is desirable not to use these channels for the frequency hopping method and instead to move to the remaining channels. Such interference may also be caused by virtue of the physical radio channel in question having already been engaged by another radio system.
In adaptive frequency hopping (AFH) methods, the channels which are subject to interference or are already engaged are suppressed automatically. There are two approaches to doing this:
In a first alternative, the mobile radio connection to be aligned, for example a Bluetooth connection, is influenced such that no interference is produced for another radio connection, such as a wireless local area network (WLAN) connection.
In a second alternative, the radio connection to be aligned is altered such that possible interference by another connection has as little unwanted interfering influence as possible.
In the second case, this can be done using, by way of example, an assessment of the bit error rate or packet error rate on the channels. In the first case, by way of example, the reception field strength can be determined at times at which the radio connection to be aligned is not active, so as to obtain information about the use of these channels by another radio system or about the presence of other unwanted sources of channel interference.
An adaptive frequency hopping method of the type outlined above, which, moreover, works in a time division multiplexing mode, involves transmission and reception only at particular times in “timeslots”. In this case, it is usual for reception slots and transmission slots to alternate. In principle, the periods, that is to say the length of the individual timeslots, are of the same length. However, in packet oriented transmission methods, which allow higher data rates, there may also be packets for which the reception or transmission period extends over a plurality of slots. These are called multislot packets.
To implement the first alternative, outlined above, of channel qualification, it is normally assumed that transmission or reception is not permanent, that is to say that data are not exclusively interchanged at the highest data rate, but rather that there are also timeslots which are unused for user data transmission. In this case, these timeslots for which it is known that no data interchange is intended to take place at this point could be used to determine the current field strength of the channel. However, this method has the drawback that the receiver in the radio system needs to be on at the times at which the field strength is intended to be determined. Beforehand, it is additionally necessary to activate the reception-end frequency synthesizer, which also needs to have locked onto the currently active channel frequency at which the evaluation is to take place. In the case of a receiver with a homodyne architecture, the synthesizer needs to lock precisely onto the reception carrier frequency, and in the case of a heterodyne system, which operates at an intermediate frequency, or in the case of a “low IF system”, in which the intermediate frequency is particularly low, the evaluation is naturally performed on a channel which is offset by the intermediate frequency. This additional locking of the reception-end synthesizers, normally implemented as a phase control arrangement, and the subsequent activation of the components required for determining field strength in the receiver results in a significant increase in the power consumption, however. This is a disadvantage particularly when, as is usual in modern radio systems, at least one remote station is in the form of a mobile appliance and is battery powered.
If the field strength is supposed to be determined in this outlined manner even at a very high data rate, that is to say with practically exclusive utilization of all the available timeslots by user data, the data rate which can effectively be attained would be reduced with further disadvantage.
To ensure the change between the physical frequencies from timeslot to timeslot in a frequency hopping method, it is usual to provide a predetermined period within each timeslot in which, when a data block's reception or transmission phase has taken place, the reception-end synthesizer can lock onto the new frequency which is desired for the subsequent timeslot.
It is an object of the present invention to specify a method for determining field strength which can be used in a mobile radio system which uses time division duplexing and a frequency hopping method and which has a relatively low power requirement without reducing the effective data rate which can be attained.